Geochemical computer models

Once formed, H2S° would partially dissociate into HS- and H+. A small amount of H2As03 and H+ would form from the dissociation of H3As03°. Some H3As03° and H2S° could also react to produce thioarsenic species, such as AsS(OH)HS (see also Section 2.7.3). Depending on the accuracy and completeness of their thermodynamic databases, geochemical computer models may be able to identify the major reactions and estimate the activities of their products. 

This chapter has discussed the fundamental importance of the master variables ps and pH in determining species distribution under the conditions prevailing in natural systems (pe —10 to - - 17 and pH 4 to 10). The equilibria contained in Sections 3.2.2 3.2.4 also provide the basis for many of the currently available geochemical computer models (e.g. WATEQ4F, PHREEQC, and MINTEQA2 ), which are being... 

Overall, geochemical computer models can be extremely useful in the description of chemical equilibria occurring in the aquatic environment. In some cases, predictions about reaction kinetics and transport of species can also be made. The application of geochemical models is not limited to natural aquatic systems but has been usefully extended to predict the eflfectiveness of certain remediation strategies in the treatment of waters emanating from contaminated sites." ... 

Geochemical computer modeling and the abundance of limestone indicates saturation of the groundwater with respect to anhydrite and calcite. Calcium, sulfate alkalinity, and TDS... 

The other approach employs a geochemical computer model, such as PHREEQC (Parkhurst 1995 also Chap. 15) with an input of a complete seawater analysis. Such a model will then calculate the activity coefficients and the species distribution of the solution according to the complete analysis and the constants of the thermodynamic database used. These constants are well known with an accuracy which is usually better than the accuracy of most of our analyses at least for the major aquatic species. Together with the real constant of the solubility product a reliable saturation index (SI = log Q) is then calculated. The constants of solubility products are not accurately known for some minerals, but for calcite, and also for most other carbonates, these constants and their dependence on temperature and pressure are very well documented. 

The geochemical computer models, EQ3/6 (Wolery 1978), SOLMINEQ.88 (Kharaka et al. 1988), PATHARC.94 (Perkins and Gtmter 1995) and TOUGHREACT (Xu et al. 2000, 2004) have been used to model water-rock reactions driven by the formation of carbonic acid when waste CO2 is injected into deep aquifers using experimentally determined rate data (e.g., Gunter and Perkins 1993). These calculations have shown that carbonate aquifers are limited in the quantity of CO2 that can be trapped, while siliciclastic aquifers have superior potential for trapping CO2 through the precipitation of carbonates, particularly,... 

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